The proposed studies are based on the central hypothesis that a series of endothelial modifications must occur in allografts to facilitate the successful development of allograft rejection. Currently, the role of endothelia in allograft rejection is poorly understood. We will investigate changes in endothelial phenotype and function at a sponge or cardiac graft site and relate them to the T cell composition of the graft infiltrate. Specifically, we will 1) demonstrate the phenotypic changes in vascular endothelia that develop in allografts during rejection. We will use selected endothelial-reactive monoclonal antibodies and immunohistologic techniques to define patterns of endothelial change in sponge and cardiac grafts. These experiments are designed to test the hypothesis that graft endothelia undergo at least three stages of differentiation, corresponding with their ability to acquire alloantigen- conditioned T cells and/or precursor T cells. 2) demonstrate the changes in endothelial affinity for T lymphocytes that develop in allografts during rejection. We will use ex vivo and in vivo adhesion assays to test the hypothesis that different mechanisms are operative in allografts for endothelial binding of activated T cells versus precursor T cells. Operative ligand/receptor systems will be identified by inhibition of ex vivo adhesion with monoclonal antibodies to known adhesion molecules. 3) determine if there is differential in vivo distribution of graft-reactive CTL, Th1 and Th2 in allograft recipients, and if these T cell subsets have differential endothelial binding capacities. We will use newly developed LDA techniques to test the hypothesis that alloantigen- conditioned T cells, but not precursor T cells, segregate anatomically in graft recipients according to inducer or effector function. We will then use cloned T cells and ex vivo adhesion assays to determine if regionalized lymphocyte-endothelial interactions are responsible for this anatomic segregation of T lymphocyte subsets. 4) determine if selected immunosuppressive drugs which interfere with T cell accumulation at a graft site also interfere with the phenotypic or functional modification of graft endothelia. We will use the above techniques to test the hypothesis that Cyclosporine and dexamethasone act by interfering with inflammatory modification of graft endothelia. In general, the proposed studies will demonstrate the contribution of endothelia to the localization of donor-specific T cells and in allograft recipients. In so doing, it will provide new information on the in vivo immunologic events associated with allograft rejection.